The present invention relates to the field of transmitter power amplifiers. More specifically, the present invention relates to the field of energy conservation and linearity improvements in transmitter power amplifiers for wireless communications systems.
The demands for low cost and power efficient wireless local-area network (WLAN) radios capable of providing a wide range of data rates and operating modes have recently accelerated. This has put more emphasis on achieving low cost radios with low power consumption. Particularly with requirements for integrated radios that operate with both direct-sequence spread spectrum (DSSS) coded waveforms and orthogonal frequency-division multiplexing (OFDM) waveforms as found in the IEEE 802.11a, b, g, and h standards, the transmitter power amplifier should be capable of efficient operation in several transmission modes and at several transmit power levels. The DSSS PSK and complementary code keying (CCK) waveforms of the IEEE 802.11b standard typically have lower peak to average ratio envelope distributions than the OFDM waveforms required in IEEE 802.11a and g standards. Moreover, efficient transmissions of OFDM waveforms at different data rates may utilize different amounts of power-amplifier backoff from the compression point to maintain optimum output power and signal fidelity. Transmitted OFDM waveforms for IEEE 802.11a and g modes can have peak-to-average ratios of 12 dB or more in the highest data rate modes. The later modes typically require a greater amount of linearity for a given output power than for the lower data rate modes.
Historically, RF power amplifiers have had to accommodate a smaller range of requirements. This allowed them to be implemented with fixed voltage and current bias techniques. Typical IEEE 802.11b xe2x80x9cWireless Fidelityxe2x80x9d (WiFi) radios utilize a fixed-bias class-A or class-AB biased RF power amplifier. Similarly, for IEEE 802.11a OFDM radios, a fixed bias class A or highly biased class-AB RF power amplifier is common. Due to the linearity demands of OFDM transmission at higher data rates, if fixed bias is used, the power amplifier output capability must be sized larger in order to transmit at similar power levels as with the less demanding IEEE 802.11b modulation modes.
A commonly applied technique to improve linearity in power amplifiers is to increase the quiescent current so that the output transistor operates in a more linear portion of the I/V curve. However, higher quiescent current also results in greater power consumption during times when the power amplifier is transmitting. For maximum efficiency in a class-AB biased amplifier, the bias current and voltage should be selected carefully for transmit mode and output power level required. Improved efficiency results if the quiescent current and power-amplifier supply voltages are varied dynamically with the instantaneous power (envelope) of the transmitted signal. But optimizing the bias current and voltage for each signal level is difficult in a highly integrated radio product that is constrained by integrated-circuit (IC) technology cost and power efficiency considerations.
Improved power amplifier characteristics (e.g., linearity and maximum power output) and efficiency generally accrue when the IC process technology is well matched to the frequency range and circuit requirements of the power amplifier. Unfortunately, highest power amplifier performance levels can best be achieved with processes that also tend to be more costly and limited in circuit integration capabilities, e.g., gallium-arsenide (GaAs) metal semiconductor field-effect transistor (MESFET) and heterojunction bipolar transistor (HBT) technologies. This has inhibited the use of sophisticated biasing and bias management circuits within the RF power amplifier ICs in the highest performance applications, which could benefit from such circuits, because they could not be integrated easily due to device and cost constraints of the RF power amplifier IC processes.
On the other hand, the RF transceiver and radio ICs are implemented today as complex mixed signal circuits. These systems are desirably highly integrated in minimum cost IC processes, e.g., complementary metal-oxide semiconductor (CMOS) or bipolar complementary metal-oxide semiconductor (BiCMOS) IC processes.
Accordingly, it is an advantage of the present invention that a bias-management system and method for a programmable RF power amplifier is provided.
It is another advantage of the present invention that a programmable bias-management system is provided that optimizes the performance of an RF power amplifier for multiple radio operating modes.
It is another advantage of the present invention that a bias-management system is provided in which a reference transistor of a bias regulator and an output transistor of a power amplifier are fabricated upon a common integrated-circuit substrate to provide close thermal coupling between the bias regulator and the power amplifier.
It is another advantage of the present invention that a bias-management system is provided in which a bias current for a power amplifier is dynamically adjusted as a function of an envelope of an input signal.
It is another advantage of the present invention that a bias-management system is provided in which a bias current for a power amplifier is dynamically adjusted to compensate for signal rectification within an output stage of the power amplifier.
The above and other advantages of the present invention are carried out in one form by an RF power-amplifier bias-management system incorporating a digitally programmed controller configured to according to a selected radio operating mode, a current-mirror circuit coupled to the controller and configured to produce a reference current as a function of the radio operating mode, a bias regulator coupled to the controller and the current-mirror circuit and configured to respond to the reference current, and a power-amplifier output stage coupled to the bias regulator and having a bias current determined by the bias regulator.
The above and other advantages of the present invention are carried out in another form by a method of adaptively controlling operation of an RF power amplifier. The method incorporates selecting a radio operating mode, producing a reference current in response to the selecting activity, establishing a bias current for an output stage of the power amplifier in response to the producing activity, and determining a supply voltage for the power amplifier in response to the selecting activity.